Electromagnetic wave filters comprising a transparent substrate, and a dielectric layer and a metallic layer which are laminated thereon alternately in this order from the substrate side are known as electromagnetic wave filters transparent in the visible region which comprise a transparent substrate, such as a glass plate, having formed on one surface thereof an electromagnetic wave shield film. Such an electromagnetic wave shield film used for this purpose is a film obtained by coating or covering an electrically conductive substance (having a small surface resistance) on a substrate in order to shield electromagnetic waves. For example, a laminate of a thin dielectric film of, e.g., a transparent metal oxide and a thin film of silver is known as a conductor film having a low resistance while securing a high visible light transmission.
JP-A-5-42624 discloses a glass plate coated with an infrared ray shield film having a structure that a silver layer is sandwiched between a pair of dielectric layers. The JP-A discloses that dielectric layers, which protect the silver layer, each use a laminate comprising at least two layers of a zinc oxide layer and a tin oxide layer. The JP-A also discloses that the infrared ray shield film can be used to windows or electronic components as an electromagnetic wave shield film.
JP-A-9-85893 discloses a laminate, formed on a glass substrate, comprising a silver layer containing 0.3 atomic % or more of palladium as a metallic layer and a zinc oxide layer containing a metal such as aluminum as a dielectric layer, wherein addition of aluminum, etc., to the zinc oxide layer reduces the internal stress to improve the adhesion to the silver layer. It also discloses a laminate of five-layered structure (two silver layers are used) having improved resistance to moist heat.
JP-A-8-104547 discloses an infrared ray shield film for an insulating sealed double-glazed unit, which is laminate of a five-layered structure having two silver layers as metallic layers and a tin oxide layer or a zinc oxide layer as dielectric layers, i.e., a structure of dielectric layer/silver layer/dielectric layer/silver layer/dielectric layer. According to the disclosure, the thickness of the dielectric layers are adjusted to make transmitted light and reflected light colorless for use on window glass.
A plasma display panel is known as a large-sized image display, but requires a powerful plasma discharge for achieving a high bright display. The plasma display panel therefore emits near infrared rays together with electromagnetic waves from the discharge region toward the front of the panel. It has been pointed out that electromagnetic waves have a possibility of giving adverse influences to a human body. The infrared rays emitted may be detected by the receptor of a remote control of appliances near the plasma display panel, so that an incorrect action may occur to turn the switch on. Various methods for preventing this have been investigated.
To overcome the above problems, it is proposed to provide a transparent material having a electromagnetic wave shield performance in front of the plasma display panel. In this connection, a method is investigated, which comprises attaching an electromagnetic wave filter comprising a dielectric layer and a silver layer which are alternately laminated, to the front of a plasma display. Such an electromagnetic wave filter is required to have the following performances:
(1) a shielding effect against electromagnetic waves.
(2) a low transmission to heat rays of the near infrared region (region of 800 to 900 nm), which are used for remote control of appliances by a remote control switch, thus having a performance for preventing an incorrect action of the appliances near the plasma display panel.
(3) a high visible light transmission to secure a bright image display.
(4) a sufficient durability against moist heat, etc., since the filter is used in the state exposed in air.
In order to satisfy both the performances (2) and (3) above, it is necessary to design the electromagnetic wave shield film so as to have the characteristics as a so-called optical low path filter, i.e., to have a high transmission at a visible light region and a low transmission at a near infrared light region, namely, showing a rapidly low transmission at the border of the two regions. This has led the concept for overcome the problem in the present invention.
JP-A-5-42624 discloses an infrared ray shield film on a glass plate, wherein a dielectric layer to be a protective layer comprises a laminate of at least two layers of a zinc oxide layer and a tin oxide layer. Therefore, durability such as resistance to moist heat resistance is excellent. However, since only one silver layer is used, the electromagnetic wave shielding performance is not sufficient. Further, even when the film is used in front of a plasma display, the film has a high infrared ray transmission in the near infrared region, making it difficult to prevent an incorrect action of a remote control of appliances near the display.
The laminate covered on the glass plate, disclosed in JP-A-9-85893 supra has improved resistance to moist heat due to that a silver layer containing 0.3 atomic % or more of palladium is used as a metallic layer, and a zinc oxide layer having added thereto a metal such as aluminum is used as a dielectric layer to reduce the internal stress and improve adhesion to the silver layer. However, since two silver layers (in five-layered structure) are used, it is difficult to obtain the requirements (2) and (3) required as the plasma display panel.
Further, regarding the infrared ray shield film disclosed in JP-A-8-104547 supra, it is a five-layered structure containing two silver layers. Therefore, it is difficult to obtain the requirements (2) and (3) for the same reason as above.